The present invention generally relates to sheetmaking control systems and, more particularly, to sheetmaking control systems wherein measuring devices scan across travelling sheets during sheet manufacture.
It is well known that on-line measurements can be made to detect properties of sheet materials during manufacture. Generally speaking, on-line measurements are made to enable prompt control of sheetmaking processes and, thus, to enhance sheet quality while reducing the quantity of substandard sheet material which is produced before undesirable process conditions are corrected. In the papermaking art, for instance, on-line sensors can detect variables such as basis weight, moisture content, and caliper of paper sheets during manufacture.
On-line measurements during sheetmaking are, however, difficult to make accurately. One reason for the difficulty is that many sheetmaking machines are large and operate at high speeds. For example, some modern paper-making machines produce sheets almost four hundred inches wide at rates of up to one hundred feet per second. Another factor which causes difficulty in making on-line measurements on sheetmaking machines is that the physical properties of sheet materials usually vary in the machine direction as well as in the cross direction. (In the sheetmaking art, the term "machine direction" refers to the direction of travel of sheet material during manufacture, and the term "cross direction" refers to the direction across the surface of a sheet perpendicular to the machine direction.)
To detect cross-directional variations in sheet materials, it is well known to use scanning sensors that periodically traverse back and forth across a sheetmaking machine in the cross direction while detecting values of a sheet property along each scan. The time required for a typical scan is generally between about twenty and thirty seconds for conventional high-speed scanners. The rate at which measurement readings are provided by such scanners is usually adjustably variable; a typical rate is about one measurement reading about every fifty milliseconds. Normally, measurement information provided by the scanning sensors is assembled for each scan to provide a "profile" of the detected property of the sheet in the cross direction. In other words, each profile is comprised of a succession of sheet measurements at adjacent locations extending generally in the cross direction. From such profiles, cross-directional variations in sheet properties can be detected. Based upon the detected cross-directional variations, appropriate control adjustments can be made to the sheetmaking machine.
In practice, the points at which scanning sensors make cross-directional measurements of travelling sheets are not aligned exactly perpendicular to the edges of the sheets being measured. Instead, because of sheet velocity, scanning sensors actually travel diagonally across the surface of moving sheets, with the result that consecutive scanning paths zig-zag relative to the sheet edges
As a result of the zig-zag paths of scanning sensors, cross-directional profiles based on sheet measurements taken by the sensors usually include machine-direction variations. In the sheetmaking art, the term MD/CD coupling is often used to describe the combining of machine-directional and cross-directional measurements. As a result of MD/CD coupling, control systems that are intended to reduce cross-directional variations can, instead, introduce artificial control disturbances which adversely affect sheet uniformity.
In conventional practice, sheetmaking control systems often do not compensate for MD/CD coupling. However, some sheetmaking control systems do attempt to reduce, or damp, the effects of machine direction variations by time-filtering techniques. Such time-filtering techniques have several shortcomings, including the fact that they necessarily entail the loss of measurement information which might otherwise be useful.
Also in conventional practice, machinedirectional variations in sheet properties are determined only at the end of each scan. The fact that machine-directional variations are calculated on a scan-by-scan basis can lead to control limitations. For example, it is not uncommon for machine-directional properties to vary such that, within the period of a single cross-directional scan, the value of a sheet property increases and then decreases. When the machine-directional properties are calculated on a scan-by-scan basis, such relatively high frequency inter-scan variations are not detected. More specifically, it can be said that machine-direction variations in sheet properties that change at a frequency greater than one-half cycle per scan cannot be detected on a scan-by-scan basis by employing conventional techniques.
The present invention, as will be described in the following, is directed to methods to accurately detect relatively high frequency machinedirectional variations in sheet properties during production.